This invention relates to improvements in protective devices for electric circuits and more particularly to an improvement in electric fuses.
The fusible elements of many electric fuses have at least one portion with a reduced cross section known as a weak spot or fusible section. The weak spots of a fusible element account for the greatest part of the electrical resistance of the element. Because the weak spot presents the greatest resistance, the heat buildup under conditions of overcurrent is greatest and most rapid in this portion of the fusible element. This rapid heat buildup causes the weak spot to fuse or melt under overcurrent conditions. As the material in the weak spot fuses, an arc forms. This arc continues to conduct current.
It is known in the art to use a magnetic field to accelerate arc extinction. This can be accomplished because as the arc continues conducting, the electric current will interact with a magnetic field which produces a force on the arc causing it to move, thus, rapidly increasing the arc length. As the arc length increases, the arc voltage quickly rises until it exceeds the system voltage. At such time as the arc voltage exceeds the system voltage, the arc current begins to decrease. Only when the arc current reaches zero is the arc actually extinguished. Thus, it can be seen that a magnetic field which causes the arc length to increase rapidly can quickly bring the arc current to zero and thereby extinguish the arc in a much shorter time.
Prior art fuses have been suggested which utilize magnetics in some form to assist in clearing. These include, for example, the fuse of U.S. Pat. No. 2,734,110 issued to Jacobs, on Feb. 7, 1956, which uses a magnetic field concentrated adjacent to the weak spot for causing the products of arcing resulting from the fusing of the weak spot to be moved outwardly through a set of orifices. U.S. Pat. No. 1,441,550 issued to Weston, on Jan. 9, 1923, discloses the use of a fuse element with two or more weak spots arranged in series and configured such that the magnetic field created by the current flow in the fuse during clearing causes the portion of the fuse located between the weak spots to be displaced sidewise. U.S. Pat. No. 3,275,771 issued to Salzer, on Sept. 27, 1966, discloses the use of parallel fuse elements with a gas-evolving insulating material disposed between them. The current flow in each of the two elements is parallel and codirectional whereby the electromagnetic forces between the parallel arcs formed by the fusing of the weak links causes the arcs to be pulled into the gas-evolving insulating material to quench the arcs. U.S. Pat. No. 4,063,297 issued to Pullen, Jr., on Dec. 13, 1977, discloses the use of a tensioned spring element in conjunction with a horn gap in which magnetic forces are used to aid in arc extinguishing U.S. Pat. No. 659,671 issued to Hewlett, on Oct. 16, 1910, discloses the use of an electromagnet in combination with a fuse element and a pair of arcing tips to extinguish the arc resulting from an overcurrent. U.S. Pat. No. 685,766 issued to Jones, on Nov. 5, 1901, discloses the use of an electromagnet in series with the fuse element to assist in extinguishing the arc.
While the fuses described in the prior art patents discussed heretofore all use magnetics in some form to extinguish the arc created in the clearing of the fuse, many require external magnetic fields or the dropping away of one or more sections of the fuse structure by gravity or spring or other force assist, causing the fuse structure to be complex and more costly.